Apparatus for fabricating single crystal

ABSTRACT

This invention relates to fabrication of a single crystal of a compound semiconductor according to the vertical Bridgman method which improves a recess in the interface between solid and melt and can obtain a stable yield of single crystal growth characterized in that a part for discharging the heat of a crucible to the outside in the radial direction is formed at least in a part in the circumferential direction of a heater part for controlling the interface between solid and melt in a heater which surrounds the crucible and a semiconductor melt is gradually solidified from a lower part to an upper part in the crucible while maintaining the interface between solid and melt in a saddle shape, thereby growing a single crystal.

This application is a Divisional application of application Ser. No.09/325,129, filed Jun. 3, 1999, now U.S. Pat. No. 6,290,773.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to method and apparatus for fabricating asingle crystal of a compound semiconductor according to the verticalBridgman method of growing a single crystal by gradually solidifying asemiconductor melt from a lower part to an upper part in a crucible.

2. Description of Related Art

In recent years, attention is being paid to the vertical Bridgman methodas a method of obtaining a large GaAs crystal having the diameterexceeding 3 inches and a low dislocation density. According to thevertical Bridgman method, a crucible for containing a material and aseed crystal (seed) is placed on the inside of a heater of a verticalelectric furnace and the material in the upper part is melt. After that,the seed is dipped into the melt and a single crystal is grown in thecrucible from a lower part to an upper part, that is, from a lower parton the seed crystal side to an upper part.

The method of growing a crystal by moving the crucible and the heaterrelatively to each other is called the VB (vertical Bridgman) method. Amethod of growing a crystal by providing a temperature gradient in whichthe temperature is high in the upper part and is low in a lower part anddecreasing the temperature as a whole while maintaining the temperaturegradient constant is called a VGF (vertical gradient freezing) method.

In case of growing a single crystal of GaAs, a method of floating B₂O₃on the surface of the material in order to prevent dissociation of As ora method of encapsulating the entire crucible in a quartz ampoule andgrowing a single crystal while maintaining the pressure at 1 atm whichis a dissociation pressure of As in GaAs in the ampoule is employed.

In any case, as a heater of a part for controlling the interface betweensolid and melt, a heater which continues uniformly without a gap in thecircumferential direction, that is, a heater having a circular shape incross section is used for the following reason. For example, when thereis variation in temperature in the circumferential direction, theinterface between solid and melt becomes flat. It is thereforeconsidered that variation in the electrical characteristics of the planeof a wafer which is cut from the crystal becomes large.

As a result of wholehearted studies of the inventors of the presentinvention, it was found that when the rounded heater is used, theinterface between solid and melt tends to become recessed as a whole andthere is a drawback that the yield of single crystal growthdeteriorates.

SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention to provide methodand apparatus for fabricating a single crystal according to the verticalBridgman method which solves the above problem of the recessed interfacebetween solid and melt and can obtain a stable yield of single crystalgrowth.

In order to achieve the object, the invention is constructed as follows.

(1) There is provided a method of fabricating a single crystal of acompound semiconductor according to the vertical Bridgman method ofgrowing a single crystal by arranging a crucible on the inner side of aheater of a vertical electrical furnace and gradually solidifying asemiconductor melt from a lower part to an upper part in the crucible,wherein a part for discharging the heat of the crucible toward theoutside in the radial direction is formed at least in a part in thecircumferential direction of a heater part for controlling the interfacebetween solid and melt in the above heater which surrounds the crucibleand the semiconductor melt is gradually solidified from a lower part toan upper part in the crucible while discharging the heat of the cruciblenot only in the vertical direction but also to the outside in the radialdirection, thereby growing a single crystal.

The vertical Bridgman method includes the vertical gradient freezingmethod (VGF method) of growing a crystal only with temperature decrease,the vertical Bridgman method (VB method) of growing a crystal byrelatively descending a growth vessel, a method of controlling the Aspressure, and a method of preventing vaporization of As by covering thesurface of the melt with B₂O₃.

When the heat of the crucible is allowed to flow not only in thevertical direction but also in the lateral direction, that is, to theoutside in the radial direction (circumferential direction), theinterface of the part which is cooled in the circumferential directionprecedes and the interface of the part facing the heater is delayed.When the shape of the interface between solid and melt is seen from thegrowth direction, therefore, the part of the recessed face and the partof the projected face mixtedly exist in the interface between solid andmelt. In the shape, a crystal defect does not easily occur and the yieldof single crystal growth largely increases as compared with the casewhere the face is recessed as a whole.

(2) There is also provided a method of fabricating a single crystal of acompound semiconductor according to the vertical Bridgman method ofgrowing a single crystal by arranging a crucible on the inner side of aheater of a vertical electrical furnace and gradually solidifying asemiconductor melt from a lower part to an upper part in the crucible,wherein a part for discharging the heat of the crucible toward theoutside in the radial direction is formed at least in a part in thecircumferential direction of a heater part for controlling the interfacebetween solid and melt in the heater which surrounds the crucible andthe semiconductor melt is gradually solidified from a lower part to anupper part in the crucible while maintaining the interface between solidand melt in a saddle shape.

When the interface is formed in a saddle shape and is seen from thegrowth direction, the part of the recessed face and the part of theprojected face mixedly exist in the interface between solid and melt. Inthe shape, the crystal defect does not easily occur and the yield ofsingle crystal growth largely increases as compared with the case wherethe interface has a recessed face as a whole.

Such a saddle-shaped interface can be obtained by forming a part fordischarging the heat of the crucible toward the outside in the radialdirection at least in a part in the circumferential direction of theheater part for controlling the interface between solid and melt in theheater surrounding the crucible. That is, by allowing the heat of thecrucible to flow not only in the vertical direction but also in thecircumferential direction (lateral direction), the interface of the partwhich is cooled in the circumferential direction precedes and theinterface of the part which faces the heater is delayed, so that thesaddle-shaped interface between solid and melt as shown in FIG. 1 can berealized.

(3) In each of the above methods, it is preferable to provide the partsfor discharging the heat of the crucible toward the outside in theradial direction at two positions in the circumferential direction inthe heater part for controlling the interface between solid and meltsymmetrically with respect to the diameter direction. Thus, theinterface can be formed in a symmetrical saddle shape.

(4) In each of the above methods, preferably, the part for dischargingthe heat of the crucible toward the outside in the radial direction ismade by gaps of divided halves of the heater part which controls theinterface between solid and melt. Although it is easy means, theinterface can be formed in a saddle shape. The gaps in thecircumferential direction of the halves of the heater are, in otherwords, the parts in which no heater exists in the circumferentialdirection and the heat of the crucible is discharged from the partswhere there is no heater. By dividing the heater part into halves, theparts for discharging the heat of the crucible toward the outside in theradial direction exist in the symmetrical positions with respect to thediameter direction, so that the interface can be formed in a symmetricalsaddle shape.

(5) In each of the above methods of fabricating a single crystal, thepart for discharging the heat of the crucible toward the outside in theradial direction can be also realized by a cooling means provided in apart in the circumferential direction of the heater part for controllingthe interface between solid and melt.

(6) There is also provided an apparatus for fabricating a single crystalof a compound semiconductor according to the vertical Bridgman method ofgrowing a single crystal by arranging a crucible on the inner side of aheater of a vertical electrical furnace and gradually solidifying asemiconductor melt from a lower part to an upper part in the crucible,wherein at least a heater part for controlling the interface betweensolid and melt in the heater which surrounds the crucible is verticallydivided into a plurality of parts and gaps for discharging the heat ofthe crucible to the outside in the radial direction are formed in a partin the circumferential direction of the heater part to allow the heat ofthe crucible to flow not only in the vertical direction but also fromthe gaps to the outside in the radial direction.

The gap in the circumferential direction of the heater is, in otherwords, a part where there is no heater in the circumferential directionand the heat of the crucible is discharged from the part where no heaterexists. Consequently, the heat of the crucible is flowed not only in thevertical direction but also in the lateral direction, that is, to theoutside in the radial direction (circumferential direction). Theinterface of the part which is cooled in the circumferential directiontherefore precedes and the interface of the part which faces the heateris delayed. As a result, in the shape of the interface between solid andmelt when it is seen from the growth direction, the part of the recessedface and the part of the projected face mixedly exist in the interface.In the shape, the crystal defect does not easily occur and the yield ofsingle crystal growth largely increases as compared with at least thecase where the interface has a recessed face as a whole.

(7) In the above apparatus, it is preferable that the heater part forcontrolling the interface between solid and melt is vertically dividedinto two parts. Although it is simple means, the interface can be formedin a saddle shape. The gap in the circumferential direction of thehalves of the heater is, in other words, a part where there is no heaterin the circumferential direction. The heat of the crucible is dischargedfrom the part where there is no heater. By dividing the heater part intohalves, the parts for discharging the heat of the crucible to theoutside in the radial direction exist in positions which are symmetricalwith respect to the diameter direction, so that the interface can beformed in a symmetrical saddle shape.

(8) There is also provided an apparatus for fabricating a single crystalof a compound semiconductor according to the vertical Bridgman method ofgrowing a single crystal by arranging a crucible on the inner side of aheater of a vertical electrical furnace and gradually solidifying asemiconductor melt from a lower part to an upper part in the crucible,wherein cooling means for discharging the heat of the crucible to theoutside in the radial direction are provided at positions symmetricalwith respect to the diameter direction at least in a part in thecircumferential direction of the heater part for controlling theinterface between solid and melt in the heater which surrounds thecrucible, so that the heat of the crucible flows not only in thevertical direction but also from the gaps to the outside in the radialdirection.

By providing the cooling means as mentioned above, the heat of thecrucible can be allowed to flow not only in the vertical direction butalso in the lateral direction, that is, to the outside in the radialdirection (circumferential direction). The interface of the part whichis cooled in the circumferential direction precedes and the interface ofthe part which faces the heater is delayed. Consequently, the interfacebetween solid and melt is formed in a shape where the part of therecessed face and the part of the projected face mixedly exist when itis seen from the growth direction. In the shape, the crystal defect doesnot easily occur and the yield of single crystal growth largelyincreases at least as compared with the case where the interface has arecessed face as a whole. The cooling means can be made by a part wherea water-cooled tube is arranged or a part where an insulating materialis removed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view showing a divided heater of a fabricatingapparatus according to an embodiment of the invention and the shape ofthe interface between solid and melt;

FIG. 2A is a vertical section of a single crystal fabricating apparatusaccording to the embodiment of the invention; and

FIG. 2B is a transverse cross section taken along line B—B of FIG. 2A.

MODES FOR CARRYING OUT THE INVENTION

The present invention will be described hereinbelow on the basis of anembodiment shown in the drawings.

A case of using a VGF furnace shown in FIGS. 2A and 2B and growing anon-doped GaAs single crystal having the diameter of 3 inches inaccordance with the VGF method will be described here as an embodiment.

A seed crystal 3 is arranged at the lower end of a crucible 7 made ofPBN. The crucible 7 contains a GaAs polycrystal as a polycrystalmaterial of a Group III-V compound. The crucible 7 is placed on acrucible supporting stand in a chamber 5 and is installed on the innerside of a heater 6 of a vertical electric furnace.

The heater 6 of the vertical electric furnace is made by a cylindricalheater arranged outside the crucible 7 so as to surround the crucible 7.The heater 6 has three stages in the vertical direction to form apredetermined temperature distribution having a temperature gradient inwhich the temperature is lower in a lower part than an upper part. Thatis, the heater 6 is constructed by: a heater 6 a for controlling theposition of the interface between solid and melt, which is placed in thecenter part serving as a heater part for controlling the interfacebetween solid and melt and constructs the interface heating unit; aheater 6 b for forming a melt, which is placed in the upper part andconstructs a high-temperature heating part; and a heater 6 c, which isplaced in a lower part and constructs a low-temperature heating part.

In the heater 6, the heater 6 a for controlling the position of theinterface between solid and melt which is placed in the central part asa heater part for controlling the interface between solid and melt isvertically divided into two, that is, right and left parts which aredivided pieces 61 and 62. Gaps 63 for discharging the heat of thecrucible to the outside in the radial direction are formed between thedivided pieces 61 and 62. The heat of the crucible 7 is thereforeallowed to flow not only in the vertical direction but also in thedirection toward the outside in the radial direction from the gaps 63 asshown by the arrows in FIG. 1.

The crystal is grown in such a manner that the GaAs polycrystal in thecrucible 7 is melted by the heater 6 to form a GaAs melt 2 as a melt ofthe Group III-V compound and the heater 6 is gradually cooled down,thereby solidifying and growing a GaAs crystal 1 as a single crystal ofthe Group III-V compound having the same orientation as that of the seedcrystal 3 from a lower part to an upper part in the crucible 7. A liquidencapsulating agent B₂O₃ 4 is charged on the GaAs melt 2, therebypreventing the dissociation of the GaAs melt 2.

As mentioned above, the heater 6 a for controlling the interface betweensolid and melt is vertically divided to form the gaps 63 as parts inwhich there is no heater in the circumferential direction of the heater6 a, and the heat is escaped or discharged from the gaps 63 (twopositions which are symmetrical with respect to the diameter direction).In this manner, the heat can be flowed not only in the direction fromthe upper to the lower but also in the circumferential direction(lateral direction).

When the GaAs single crystal is grown by using the fabricating apparatushaving such a configuration, the interface of the part which is cooledin the circumferential direction precedes and the interface of the partwhich faces the heater is delayed. Consequently, the interface betweensolid and melt has a saddle shape as shown in FIG. 1. That is, in theshape of FIG. 1, a recessed face shown by A-E-C and a projected faceshown by B-E-D are formed in the directions which perpendicularly crosseach other.

When the interface has such a saddle shape and is seen from the growthdirection (from above in FIG. 1), the recessed face part and theprojected face part mixedly exist on the interface between solid andmelt. In the shape, a crystal defect occurs less and the yield of singlecrystal growth largely increases at least as compared with a case wherethe interface between solid and melt has a recessed face as a whole.

It was found that, strangely, the degree of recess of the most recessedpart (E part in FIG. 1) when the interface is formed in a saddle shapeby using the heater 6 which is divided into halves becomes low ascompared with that of the case where the rounded heater is used. Thatis, variation in the electrical characteristics of the plane when awafer is formed becomes small as compared with that in a crystal grownby using a conventional rounded heater.

The optimum value of the width of the part where there is no heater inthe circumferential direction, that is, the width of the gap 63 betweenthe divided pieces 61 and 62 cannot be easily expressed by a numericalnumber. When it is too large, the recess and projection of the saddleshape become too large. When it is too small, the saddle shape cannot beobtained. The optimum value of the gap 63 changes according to thecrystal size, that is, the heater size.

In any case, when the heater 6 a divided into halves is used, the recessin the interface between solid and melt is improved. Consequently,advantages such that the yield of single crystal growth improves largelyand a distribution of the electrical characteristics of the cut wafersurface becomes uniform can be obtained.

Embodiment

An embodiment will be described with reference to FIGS. 2A and 2B. Theseed crystal 3, 3000 g of the GaAs material, a dopant Si and 200 g ofB₂O₃ 4 were charged into a PBN crucible 7 having the inner diameter of80 mm. The crucible 7 was set in the electrical furnace. The heater 6has an inner diameter of 120 mm and is divided into halves. The widthof the gap 63 where there is no heater was approximately 20 mm.

After N₂ gas substituted for the atmosphere gas in the chamber 5, thetemperature was increased. The temperature gradient was adjusted to 4°C./cm, the material was melted, and the seed was dipped into the melt.After that, the temperature was decreased at 1° C./h to grow a crystal.After completion of the growth, the crystal was cooled to the roomtemperature at 25 to 100° C./h and then taken out.

As a result of similarly carrying out the growing operation ten times,the yield of single crystal growth reached 90%. When each of thosesingle crystals was vertically sliced and the interface between solidand melt was observed by striation, it was found that the interface hasa saddle shape as shown in FIG. 1.

When a wafer was cut from the crystal and variation in the carrierconcentration of the plane was measured, it was found that the carrierconcentration was at the level of 1.0×18¹⁸ cm⁻³ and the variation waswithin ±3%. It was found that the value was improved as compared withthe following comparative example.

Comparative Example

A crystal was grown under that same conditions as those of theembodiment except that a rounded heater was used. As a result ofrepeatedly carrying out the growing operation ten times, the yield ofsingle crystal growth was 60%. As a result of observing the interfacebetween solid and melt, it was found that the recessed face was large asa whole. It was also found that a defect easily occurs from the part ofmeniscus which is in contact with the crucible by the influence.

When a wafer was cut and the carrier concentration of the plane wasmeasured, it was found that the carrier concentration was at the levelof 1.0×18¹⁸ cm⁻³ and the variation was +10%.

Modification

The present invention is not limited to the foregoing embodiment butalso can be modified as below.

(1) All of compound semiconductors in addition to GaAs can be applied asa material of the crystal. As examples of the semiconductor materialswhich can be applied to the invention, Group III-V compounds such asGaAs, InAs, GaSb, InSb, GaP and InP, Group II-VI compounds such as CdTe,ZnSe, ZnS, and HgTe, Group IV elements such as Si and Ge, and mixedcrystals each including one or more kinds of the above elements.

(2) Either the VB method or the VGF method can be applied.

(3) Although it is proper that the number of the vertical division ofthe heater, that is, the division in the circumferential direction ofthe heater is two, division of up to about four is considered to beeffective to form the interface between solid and melt in the saddleshape. There is no point in dividing the heater more finely from theviewpoint of forming the interface between solid and melt in the saddleshape.

(4) Since the heater 6 is divided into halves in order to discharge theheat of the crucible from the parts in the circumferential directionwhere there is no heater (parts corresponding to the gaps 63) toward theoutside in the radial direction, even if the rounded heater is used, byproviding cooling means (for example, a water cooled tube is provided, aheat insulating material is partially removed, or the like) at thesymmetrically positions in the circumferential direction, the heat canbe also discharged in the circumferential direction (lateral direction).This belongs to the scope of the present invention.

(5) As mentioned above, since the optimum value of the width of the partwhere there is no heater in the circumferential direction of the heater(the part where the gap 63 exists or the part where the cooling means isarranged) changes according to the size of the single crystal to beobtained (the diameter of the heater 6) and the material and shape of aninsulating material when a part where the insulating material on thecircumferential face of the heater is removed as cooling means, itcannot be decided unconditionally. In any case, however, when the heater6 a which is divided into halves is used, the recessed plane of theinterface between solid and melt is improved.

Consequently, advantages such that the yield of single crystal growth islargely improved and the distribution of the electrical characteristicsof the plane of the cut wafer becomes uniform can be obtained.

As described above, according to the invention, the following excellenteffects can be obtained.

(1) According to the method of fabricating a single crystal of theinvention, the part for discharging the heat of the crucible toward theoutside in the radial direction is formed at least in a part in thecircumferential direction of the heater part for controlling theinterface between solid and melt in the heater which surrounds thecrucible and the single crystal is grown while discharging the heat ofthe crucible not only in the vertical direction but also to the outsidein the radial direction. Consequently, the interface of the part whichis cooled in the circumferential direction precedes and the interface ofthe part facing the heater is delayed. When the shape of the interfacebetween solid and melt is seen from the growth direction, therefore, thepart of the recessed face and the part of the projected face mixtedlyexist in the interface between solid and melt. In the shape of theinterface between solid and melt, therefore, a crystal defect does noteasily occur and the yield of single crystal growth largely increases ascompared with the case where the interface is recessed as a whole. Thedistribution of the electric characteristics of the plane of the cutwafer becomes uniform.

(2) According to the method of fabricating a single crystal, the partfor discharging the heat of the crucible toward the outside in theradial direction is formed at least in a part in the circumferentialdirection of the heater part for controlling the interface between solidand melt in the heater which surrounds the crucible and the singlecrystal is grown while maintaining the interface between solid and meltin a saddle shape. Consequently, when it is seen from the growthdirection, the part of the recessed face and the part of the projectedface mixedly exist in the interface between solid and melt. In the shapeof the interface between solid and melt, therefore, the crystal defectdoes not easily occur and the yield of single crystal growth largelyincreases as compared with the case where the interface is recessed as awhole. The distribution of the electric characteristics of the plane ofthe cut wafer becomes uniform.

(3) According to the apparatus for fabricating the single crystal, atleast the heater part for controlling the interface between solid andmelt is vertically divided into a plurality of parts in the heatersurrounding the crucible, the gaps for discharging the heat of thecrucible to the outside in the radial direction are formed in a part ofthe heater part in the circumferential direction, and the heat of thecrucible is allowed to flow not only in the vertical direction but alsofrom the gaps to the outside in the radial direction. Consequently, theinterface of the part which is cooled in the circumferential directionprecedes and the interface of the part facing the heater is delayed. Asa result, the interface between solid and melt has a shape in which thepart of the recessed face and the part of the projected face mixedlyexist when it is seen from the growth direction. In the shape of theinterface between solid and melt, the crystal defect does not easilyoccur and the yield of single crystal growth largely increases ascompared with a case where the interface between solid and melt isrecessed as a whole. The distribution of the electric characteristics ofthe plane of the cut wafer becomes uniform.

What is claimed is:
 1. An apparatus for fabricating a single crystal ofa compound semiconductor according to a vertical Bridgman method ofgrowing a single crystal by arranging a crucible on an inner side of aheater of a vertical electrical furnace and gradually solidifying asemiconductor melt from a lower part to an upper part in the crucible,wherein at least a heater part for controlling an interface betweensolid and melt in said heater which surrounds the crucible is verticallydivided into a plurality of parts and gaps for discharging heat of thecrucible to outside in a radial direction are formed in a part in acircumferential direction to allow the heat of the crucible to flow notonly in a vertical direction but also from said gaps to the outside inthe radial direction.
 2. An apparatus according to claim 1, wherein theheater part for controlling said interface between solid and melt isvertically divided into two parts.
 3. An apparatus according to claim 1,wherein the gaps, which are formed in the part in the circumferentialdirection, extend vertically.
 4. An apparatus for fabricating a singlecrystal of a compound semiconductor according to a vertical Bridgmanmethod of growing a single crystal by arranging a crucible on an innerside of a heater of a vertical electrical furnace and graduallysolidifying a semiconductor melt from a lower part to an upper part inthe crucible, wherein cooling means for discharging heat of the crucibleto outside in a radial direction are provided at positions symmetricalwith respect to a diameter direction at least in a part in acircumferential direction of a heater part for controlling the interfacebetween solid and melt, so that the heat of the crucible flows not onlyin a vertical direction but also from said gaps to the outside in theradial direction.
 5. An apparatus according to claim 4, wherein thecooling means is a part where a water-cooled pipe is arranged or a partwhere an insulating material is removed.